Sampling module and a method of sampling one or more analogue characteristics of a power transmission system

ABSTRACT

A sampling module, for sampling one or more analog characteristics of a power transmission system, including at least one input circuit for sampling a respective analog characteristic. Each input circuit includes: a scaling circuit for reducing the magnitude of the analog characteristic to a desired level; an isolating circuit for creating an electrical barrier between respective upstream and downstream portions of the input circuit; and an analog to digital converter for digitizing the analog characteristic to produce a digital data stream. The scaling circuit is electrically connected to an input of the analog to digital converter, and the isolating circuit is electrically connected directly to an output thereof.

This invention relates to in particular, but not exclusively, a samplingmodule and a method of sampling one or more analogue characteristic of apower transmission system.

An intelligent Electronic Device (IED) uses samples of the analoguevoltage and current characteristics of a power transmission system tohelp protect and control the system.

A conventional arrangement for sampling such analogue characteristics isa, so called, “multiplex system”, as shown schematically in FIG. 1.

A typical multiplex system 10 includes a plurality of inputs 12, eachfor sampling a respective analogue characteristic.

Each input 12 includes an isolating transformer 14 and an anti-aliasingfilter 16 electrically connected in series therewith.

An output of each anti-aliasing filter 16 is connected to an analoguemultiplexer 18 which is, in turn, connected in series with a hold filter20.

The hold filter 20 is connected in series with an analogue to digitalconverter (ADC) 22.

In use, each isolation transformer 14 steps down, i.e. reduces, themagnitude of the sampled analogue characteristic to a level suitable foranalogue to digital conversion by the ADC. In addition, each isolatingtransformer 14 isolates the remaining portion of the multiplex system10.

Isolation is required because each input 12 is connected via a step downtransformer (not shown) to the power transmission system operating at ahigher voltage.

The isolating transformer provides protection against voltages inducedin the wiring and differences in earth potential that may exist betweenthe location of the step down transformer and the location of the IED.The isolation provided also protects a human operator and the variousother components of the system from the high voltages at the inputs 12.

The analogue characteristic is fed from each isolating transformer 14through a corresponding anti-aliasing filter 16 to remove noise and highfrequency harmonics.

The analogue multiplexer 18 then feeds each filtered analoguecharacteristic sequentially to the hold filter 20 which stabilises therespective analogue characteristic. This allows the ADC 22 to digitiseeach characteristic before outputting a digital data stream to aprocessing module 24.

There are a number of disadvantages associated with the aforementionedmultiplex system 10.

Each of the isolating transformers 14 needs to be physically large inorder to accurately reproduce the analogue characteristic. As aconsequence the system 10 is large and heavy.

In addition, each of the isolating transformers 14 has a limited linearoperating range. This results in non-linearity errors which are often attheir most severe when the greatest accuracy is needed.

Furthermore, each isolating transformer 14 tends to leak flux.Consequently, close packing of the isolating transformers 14 leads tocross-talk between adjacent transformers 14.

Therefore it is a general aim of the invention to provide a samplingmodule which is more compact and more accurate than conventionalmultiplex sampling systems.

According to a first aspect of the invention there is provided asampling module, for sampling one or more analogue characteristics of apower transmission system, comprising at least one input circuit forsampling a respective analogue characteristic, the or each input circuitincluding:

a scaling circuit for reducing the magnitude of the analoguecharacteristic to a desired level;

an isolating circuit for creating an electrical barrier betweenrespective upstream and downstream portions of the input circuit; and

an analogue to digital converter for digitising the analoguecharacteristic to produce a digital data stream,

the scaling circuit, the isolating circuit and the analogue to digitalconverter being electrically connected in series.

The inclusion of separate scaling and isolating circuits allows foroptimisation of the scaling and isolating functions while obviating theneed for conventional isolating transformers.

This results in a sampling module which is more compact and lighter thana conventional multiplex system. The invention is also able to providemore accurate sampling because it permits the reduction of cross talkand non-linearity errors.

In addition, the inclusion of an analogue to digital converter (ADC) ineach input circuit allows for the concurrent sampling of a plurality ofanalogue characteristics, thereby eliminating the bottleneck associatedwith the analogue multiplexer 18 of conventional multiplex systems 10.

Furthermore, concurrent sampling obviates the skew introduced inconventional multiplex systems 10 when samples can only be takenconsecutively.

Concurrent sampling also makes it possible to configure differing samplerates for respective analogue characteristics according to the degree ofresolution required.

Preferably the scaling circuit is electrically connected to an input ofthe analogue to digital converter and the isolating circuit iselectrically connected to an output thereof.

Isolating the digital data stream in this way reduces the likelihood ofany external noise corrupting the data stream.

In addition, isolating the digital data stream also means that it is notnecessary to faithfully reproduce the digital data when transferring itacross the electrical barrier. This is because it is only necessarydownstream of the electrical barrier to be able to discern a 0 or a 1rather than, e.g. a complete analogue waveform.

Conveniently the scaling circuit is or includes a resistor network. Thisresults in the scaling circuit having a linear response over a desiredmeasurement range, thereby helping to minimise non-linearity errors.

Optionally the resistor network defines a potential divider.

Alternatively the resistor network defines a shunt.

The foregoing arrangements allow the sampling of analogue voltage andanalogue current characteristics, respectively.

In a preferred embodiment of the invention the analogue to digitalconverter is or includes a sigma-delta modulator. The inclusion of asigma-delta modulator allows for the transfer of a single-bit word, i.e.a 0 or a 1, across the electrical barrier of the isolating circuit at ahigh frequency.

This results in the digitised analogue characteristic having a finerresolution compared to that produced by a conventional 16-bit ADC,

Preferably the isolating circuit is or includes a single pulsetransformer. Such a transformer has particularly desirable transfercharacteristics while providing a desired degree of electrical isolationdownstream thereof.

Optionally the sampling module further includes a data interfaceelectrically connected to the or each input circuit so as to allow theoutput of a respective digital data stream to a processing module.

Conveniently the downstream portion of the or each input circuitincludes a digital signal processing module.

In another preferred embodiment of the invention the digital signalprocessing module includes a digital filter.

In a further preferred embodiment of the invention the digital signalprocessing module includes a re-sampling module.

In a still further preferred embodiment of the invention the samplingmodule further includes a control interface electrically connected tothe or each input circuit so as to permit configuration of the digitalsignal processing module and the ADC by a control module.

The foregoing features permit processing of the digitised analoguecharacteristic to a desired degree before output to a processing module.

According to a second aspect of the invention there is provided a methodof sampling one or more analogue characteristics of a power transmissionsystem, comprising the steps of providing at least one input circuit forsampling a respective analogue characteristic, and electricallyconnecting in series within the or each input circuit:

a scaling circuit for reducing the magnitude of the analoguecharacteristic to a desired level;

an isolating circuit for creating an electrical barrier betweenrespective upstream and downstream portions of the input circuit; and

an analogue to digital converter for digitising the analoguecharacteristic to produce a digital data stream.

The method of the invention shares the advantages associated with theaforementioned sampling module according to the invention.

There now follows a brief description of a preferred embodiment of theinvention, by way of non-limiting example, with reference being made tothe accompanying drawings in which:

FIG. 1 is a schematic of a known multiplex sampling system;

FIG. 2 is a schematic of a sampling module according to a firstembodiment of the invention;

FIG. 3 is a more detailed schematic of the sampling module shown in FIG.2; and

FIG. 4 is a schematic of a preferred isolating circuit according to theinvention.

A sampling module according to a first embodiment of the invention isdesignated generally by the reference numeral 30.

The sampling module 30 includes a plurality of input circuits 32, eachinput circuit 32 being for sampling a particular analogue characteristicof a power transmission system (not shown) to which the sampling module30 is electrically connected.

In the embodiment shown each input circuit 32 includes a scaling circuit34 at an upstream, input end 36 thereof. The scaling circuit 34 iselectrically connected in series to an input of an analogue to digitalconverter (ADC) 22. A preferred ADC 22, in the form of a sigma-deltamodulator 38, is shown.

An output of the sigma-delta modulator 38 is electrically connected inseries to an isolating circuit 40.

In other embodiments of the invention differing series arrangements ofthe scaling circuit 34, ADC 22, and isolating circuit 40 are alsopossible.

In each input circuit 32 the scaling circuit 34 is a resistor network(not shown). The resistor network may be in the form of a potentialdivider or a shunt, for sampling analogue voltage and analogue currentcharacteristics, respectively.

Each isolating circuit 40 includes a single pulse transformer 42, asdetailed schematically in FIG. 4.

A portion of each input circuit 32 downstream from the isolating circuit40 includes a digital signal processing module 44, each of whichincludes digital filter 46 and a re-sampling module 48.

The output of each digital signal processing module 44 is electricallyconnected to a data interface which, in turn, is connectable to aprocessing module 24.

The sampling module 30 also includes a control interface 52 which iselectrically connected to each input circuit 32. The control interface52 is configured to be connectable to a control module (not shown),which permits configuration of the digital signal processing module 44and the ADC 22.

In addition, the embodiment of the sampling module 30 shown alsoincludes a master clock 54 (FIG. 3).

In use, each scaling circuit 34 scales a respective sampled analoguecharacteristic to a level suitable for analogue to digital conversion bythe corresponding sigma-delta modulator 38. It is possible to determinethe measurement range of each scaling circuit 34 by selecting the valuesof the resistors therein.

Furthermore, it is possible to improve the accuracy of each sampledanalogue characteristic by calibrating each scaling circuit 34.Calibration could correct any linear gain or offset errors introduced bya given scaling circuit 34. Calibration could also correct any inversionof the sampled analogue characteristic caused by incorrect assembly ofthe scaling circuit 34.

Each sigma-delta modulator 38 outputs a 1-bit, serial, digital datastream corresponding to a given sampled analogue characteristic. A clocksignal controls the sampling rate of each sigma-delta modulator 38.

Respective single pulse transformers 42 transfer the digital data streamin a first direction from the sigma-delta modulator 38 to a digitalsignal processor 44, while creating an electrical barrier between theserespective components. Each digital signal processor 44 may beconfigured to receive one or more digital data streams.

Each single pulse transformer 42 also transfers power to drive thecorresponding sigma-delta modulator 38, and the aforementioned clocksignal, in a second, opposite direction.

Each single pulse transformer 42 implements a forward converter topologywith a frequency of operation equal to that of the clock signal, therebyallowing the corresponding sigma-delta modulator 38 to derive the clocksignal directly from the transferred power.

Transfer of the binary signal data stream is achieved by changing theamplitude of the voltage used to reset the transformer's flux, on acycle by cycle basis, from the secondary side of the forward converter.Monitoring of the converter's primary waveform determines whether a highvoltage has been used to quickly reset the flux or a low voltage hasbeen used to reset the flux more slowly.

The digital filter 46 of each digital signal processing module 44filters a respective digital data stream to, e.g. remove any extraneousnoise.

The re-sampling module 48 of each digital signal processing module 44 isable to generate a digital data stream having a desired sampling rate.

Configuration of the digital signal processing module 44 and thesampling rate of each sigma-delta modulator 38 is conducted through thecontrol interface 52, thereby allowing for differing sample rates andfiltering characteristics in each input circuit 32 according to therequirements of an attached processing module 24, connected via the datainterface 50.

Each input circuit 32 transmits its digital data stream corresponding toa given sampled analogue characteristic, to the processing module 24 viathe data interface 50. The master clock 54 time tags each digital datastream so as to ensure that the data within a particular stream iscorrectly synchronised following transmission to the external device.

Accordingly, each input circuit 32 contributes an independently filteredsample value to a given digital data stream.

Each data stream block (44 in FIG. 3) accepts the isolated bit streamsfrom every input circuit (32 in FIG. 3), filters them to remove unwantedfrequency components and then resamples them to produce packets ofsamples at the sample rate required by the IED (24 in FIG. 2). Theparameters of the digital filter (46 in FIG. 3) and the period of theresampling logic (48 in FIG. 3) can both be configured by the IED (24),on-the-fly if necessary, to suit the particular protection algorithm. Asthe invention contains multiple concurrent data stream blocks (44), itis possible to have different protection algorithms runningsimultaneously within the IED that require sample data at differentrates and with different levels of filtering.

1-10. (canceled)
 11. An intelligent electronic device comprising: asampling module that samples one or more analog characteristics of apower transmission system, the sampling module comprising at least oneinput circuit for sampling a respective analog characteristic, the inputcircuit comprising: a scaling circuit that reduces magnitude of theanalog characteristic to a desired level; an isolating circuit thatcreates an electrical barrier between respective upstream and downstreamportions of the input circuit; an analog to digital converter thatdigitizes the analog characteristic to produce a 1-bit serial digitaldata stream, the scaling circuit being electrically connected to aninput of the analog to digital converter, and the isolating circuitbeing electrically connected directly to an output thereof; wherein thesampling module further comprises a digital signal processing module inseries with the isolating circuit and comprising a digital filter thatdigitally filters the 1-bit serial digital data stream and a resamplingmodule that re-samples the filtered data stream output from the digitalfilter, and wherein the intelligent electronic device further comprisesan additional processing module in series with the digital signalprocessing module, the digital signal processing module producingpackets of samples at a sample rate required by the additionalprocessing module, parameters of the digital filter and a period of theresampling module being both configured by the additional processingmodule to suit a particular algorithm.
 12. An intelligent electronicdevice according to claim 11, wherein the sampling module comprises aplurality of parallel input circuits and a plurality of parallel digitalsignal processing modules, an output of each input circuit beingconnected to an input of each digital signal processing module,parameters of digital filtering of the digital filters of the digitalsignal processing modules and a period of resampling of the resamplingmodules of the digital signal processing modules being configured by theadditional processing module to suit different protection algorithmsrunning simultaneously within the additional processing module thatrequire data at different rates and with different levels of filtering.13. An intelligent electronic device according to claim 11, wherein thescaling circuit is or includes a resistor network.
 14. Intelligentelectronic device according to claim 13, wherein the resistor networkdefines a potential divider.
 15. Intelligent electronic device accordingto claim 13, wherein the resistor network defines a shunt. 16.Intelligent electronic device according to claim 11, wherein the analogto digital converter is or includes a sigma-delta modulator. 17.Intelligent electronic device according to claim 11, wherein the analogto digital converter is controlled with a clock signal and the isolatingcircuit is or includes a single pulse transformer that transfers a) the1-bit digital data stream in a first direction from the analog todigital converter to the digital signal processing module and b) powerto drive the corresponding analog to digital converter and the clocksignal in a second opposite direction, the single pulse transformerimplementing a forward converter topology with a frequency of operationequal to that of the clock signal, thereby allowing the correspondinganalog to digital converter to derive the clock signal directly from thetransferred power.
 18. A processing method comprising: sampling one ormore analog characteristics of a power transmission system comprising atleast one input circuit, the sampling comprising at least one inputtingcomprising: reducing magnitude of the analog characteristic to a desiredlevel; creating an electrical barrier between respective upstream anddownstream portions of the input circuit; digitizing the analogcharacteristic to produce a 1-bit digital data stream; wherein saidsampling further comprises a digital signal processing following the atleast one inputting and comprising digital filtering the 1-bit digitaldata stream followed by resampling the filtered data stream output fromthe digital filtering, and wherein the processing method furthercomprises an additional processing, the digital signal processingproducing packets of samples at a sample rate required by the additionalprocessing, parameters of the digital filtering and period of theresampling being both configured by the additional processing to suit aparticular algorithm.
 19. A processing method according to claim 18,wherein the sampling method further comprises a plurality of parallelinputtings and a plurality of parallel digital signal processings, dataoutput by each inputting being processed by each digital signalprocessing, parameters of digital filtering and a period of resamplingof the parallel digital signal processing being configured by theadditional processing to suit different protection algorithms runningsimultaneously within the additional processing that require data atdifferent rates and with different levels of filtering.
 20. A processingmethod according to claim 18, wherein the digitizing is controlled witha clock signal and the creating an electrical barrier transfers a) the1-bit digital data stream in a first direction from an analog to digitalconverter that implements the digitizing to a digital signal processingmodule that implements the digital signal processing and b) power todrive the corresponding analog to digital converter and the clock signalin a second opposite direction, a single pulse transformer implementinga forward converter topology with a frequency of operation equal to thatof the clock signal, thereby allowing the corresponding analog todigital converter to derive the clock signal directly from thetransferred power.